1. Field of the Invention
The present invention relates to instruments for measuring and testing and, more particularly to, instruments for measuring deformation of materials.
2. Description of the Prior Art
In tri-axial compression tests lateral deformations in material test specimens are conventionally measured with either cantilevered spring arms which are equipped with electrical strain gauges or linear variable differential transformers. Both methods require that the deformeter be mounted on a "bird cage" surrounding the specimen. Lateral deformations may be difficult to measure by such means due to the fact that specimens deform not only laterally but longitudinally. It may also be difficult to account for the compressibility of the membrane which is conventionally used to surround the specimen using such methods. A disadvantage which is particularly applicable to the use of linear variable differential transformers is that they have limited range and are easily damaged during uncontrolled specimen failure.
Strain path tests in which strain gauges are adhered directly to the specimen are also conventionally used to measure lateral deformations. Such direct attachment is required due to the necessity that deformeters used in such tests have infinite resolution. Direct attachment in this manner requires that the sides of the specimen be very smooth and free of voids. Because of the requirement of such smoothness many rock and concrete materials can not be tested by this method.